If you have the laminated maps, you can mark the route in permanent (OHP type) marker, and wipe it off with meths later.

Nick,
Assuming your maps are paper, not laminated, I find yellow highlighter pen shows up best, especially outside where there's plenty of UV to make it glow. However it does wear off if you handle it with sweaty hands.
Nick Ham 19655.

What is the best method for marking-up maps? I'm about to mark-up maps for the 100 and previously, have always used a highlighter to mark a route. Is this what others do or is there another recommended method? Have seen maps marked-up using pencil crosses along the route but would be pleased to hear from others with their views. Thanks.